October 1, 2013

Rewriting The Climate History Of The Arctic Ocean

Traces of large ice sheets from the Pleistocene on a seamount off the north-eastern coast of Russia have been discovered by a team of geologists and geophysicists from the Alfred Wegener Institute (AWI) Helmholtz Centre for Polar and Marine Research.

For the first time, these traces confirm that within the last 800,000 years in the course of ice ages, ice sheets more than approximately a mile thick also formed in the Arctic Ocean. The findings, published in Nature Geoscience, suggest that the climate history for this part of the Arctic needs to be rewritten.

AWI scientists, including geologist Dr. Frank Niessen, had previously discovered the first signs of conspicuous scour marks and sediment deposits on the ocean floor during a Polarstern expedition in 2008. These traces were found north of Wrangle Island, Russia. They were unable to gather extensive proof until this last year during an Arctic expedition aboard the South Korean research vessel Araon. "After we had analyzed the bathymetric and seismic data from our first voyage, we knew exactly where we needed to search and survey the ocean floor with the swath sonar of the Araon on the second expedition," said Niessen.

The research team used the data collected to create a topographic map of the Arlis Plateau - a seamount on which deep, parallel-running furrows can be discerned on the upper plateau and the sides - and over an area of 1500 square miles and to an ocean depth of nearly 4000 feet. "We knew of such scour marks from places like the Antarctic and Greenland. They arise when large ice sheets become grounded on the ocean floor and then scrape over the ground like a plane with dozens of blades as they flow. The remarkable feature of our new map is that it indicates very accurately right off that there were four or more generations of ice masses, which in the past 800,000 years moved from the East Siberian Sea in a north-easterly direction far into the deep Artic Ocean," says Niessen.

Traditional textbook beliefs about the history of Arctic glaciation are overturned by the new findings. “Previously, many scientists were convinced that mega-glaciations always took place on the continents – a fact that has also been proven for Greenland, North America, and Scandinavia. However, it was assumed that the continental shelf region of North-eastern Siberia became exposed in these ice ages and turned into a vast polar desert in which there was not enough snow to enable a thick ice shield to form over the years. Our work now shows that the opposite was true. With the exception of the last ice age 21,000 years ago, ice sheets formed repeatedly in the shallow areas of the Arctic Ocean. These sheets were at least 1200 meters thick and presumably covered an area as large as Scandinavia," Niessen said.

The team is still unable to say for sure under what climate conditions these ice sheets form, nor exactly when they left their marks on the bottom of the Arctic Ocean. "We theorize that the East Siberian ice sheets arose during various ice ages when the average global temperature was around five to eight degrees Celsius cooler than what it is today. But evidently this relatively minor temperature difference was often sufficient to allow initially thin ocean ice to grow into an immense ice cap. An example that shows just how sensitively the Arctic reacts to changes in the global climate system," says the geologist.

The research team wants to continue their studies by collecting soil samples from deeper layers of the ocean floor with a sediment core drill and thus learn more details about the prehistoric ice sheets. "Our long-term goal is to reconstruct the exact chronology of the glaciations so that with the aid of the known temperature and ice data, the ice sheets can be modeled. On the basis of the models, we then hope to learn what climate conditions prevailed in Eastern Siberia during the ice ages and how, for example, the moisture distribution in the region evolved during the ice ages," said Niessen. The hope is that this information will help predict possible changes in the Arctic as a consequence of climate change more accurately.

Niessen and his team are predicting a variety of surprising discoveries in the Arctic Ocean in the future. "As the Arctic Ocean sea-ice cover continues to shrink, more formerly unexplored ocean area becomes accessible. Today less than ten percent of the Arctic Ocean floor has been surveyed as thoroughly as the Arlis Plateau," says the AWI geologist. Without the collaboration of the AWI research team and the South Korean Polar Research Institute KOPRI researchers, this study would not have succeeded. "We complemented each other perfectly in this research. Our South Korean colleagues had the expedition and ship time, we knew the coordinates of the area in which we now found the evidence of the mega-glaciations," says Frank Niessen.